1. Field of the Invention
This invention relates to combustible gas sensors and more particularly to catalytically activated gas sensors having a multi-layered metal oxide film structure for improving the selective detection of target gases.
2. Description of the Prior Art
U.S. Pat. No. 4,542,640 discloses a selective gas detection system in which semiconductor gas sensors are used to detect polluting, toxic, and combustible gases such as carbon monoxide, methane, hydrogen sulfide, hydrocarbons, and many other combustible vapors commonly found in ambient air. These target gases occur as impurities in an air atmosphere. Metal oxide semiconductor sensors of the N-type, such as tin oxide, zinc oxide and the like, are used to detect the target gases. The gas detecting devices disclosed in U.S. Pat. Nos. 3,676,820 and 4,457,161 are further examples of devices which detect threshold levels of target gases.
The known gas sensors adsorb the target gases and convert the concentrations of gases into an electrical signal by changing resistivity. The known semiconductor sensors of the thin film type are composed of semiconducting materials, such as metal oxide ceramics, or organic semiconductor materials and others.
With semiconductor sensors the concentration of a combustible or "target" gas is related to the change in the electrical resistance of the semiconductor film. The change in resistance is due to changes of temperature and the presence of reducing gases and solvents. In the presence of reducing gases the concentration of electrons in the surface of the metal oxide film changes due to the adsorption/desorption and reaction of ionic species at the surface of the film. The oxidation/reduction chemical reaction between adsorbed oxygen and the combustible gas yields a product that is desorbed from the surface. Upon removal of the combustible gas from the oxygen-containing atmosphere, the consumed surface-desorbed oxygen is replenished and the resistance of the metal oxide film returns to its original value.
The film surface-adsorbed oxygen will also be affected by the reactive vapor impurities in the ambient air. Water vapor, a common constituent in ambient air, is known to affect the response of a metal oxide film gas sensor in the same manner as a combustible gas. Some other reactive vapors in the air can be those of common solvent contaminants. These are normally referred to as "interference" gases, such as ethanol, methanol, isopropanol, ammonia, benzene, toluene, pentane and hexane.
The total change in the resistance of a metal oxide film sensor arises due to the change in the target combustible gases and all the interference gases that are present in the ambient air being tested. Also, it has been shown that by adding catalytic metals to tin oxide material, the response characteristics of a gas sensor will change at any given operating temperature. Another factor affecting response characteristics is the use of internal binders to hold metal oxide sensing particles in contact with the electrodes and substrate. Internal binders cause an undesireable increase in the response of the sensor to humidity. Insufficient concentration of internal binders leads to mechanical instability throughout the sensing film, causing inaccurate readings and shortened life times.
The multi-gas/multi-sensor system in U.S. Pat. No. 4,542,640 requires that the number of thick film sensors should be greater than or equal to the number of selected gases to be distinguished. Each sensor in the array of sensors should have a response characteristic which differs from the response characteristics of each of the other sensors in its response to at least one of the set of selected gases. This difference may result from changes such as film composition and film gas sensor temperature that can be adjusted in a multi-sensor array to obtain the different responses for each sensor.
The responses of each gas sensor in the above described array are measured for each of the selected gases and the combinations of the selected gases. The measured responses are applied to a system of gas response equations in which each response is substituted in the specific equations and the values of the constants are determined. The equations are then solved for the concentration of the gases in the target gas mixture.
The accuracy of the multi-gas/multi-sensor approach disclosed in U.S. Pat. No. 4,542,640 depends on the extent of the gas response data of all the combinations of the reactive gases in the mixture. For a multi-component gas mixture, as often encountered, the collection of gas sensor data with all the possible combinations of gases can be very time consuming. This limits the collection of data to the single, and some of the important binary combinations. As a result the accuracy in the calculated gas concentrations will then be compromised to the extent of the contribution of those responses that were eliminated. In order to use the multi-gas/multi-thick film sensor system disclosed in U.S. Pat. No. 4,542,640 and maintain the necessary simplicity and accuracy that is demanded of a commercial gas monitoring instrument one must increase the relative gas selectivity from the appropriate choice of film compositions and film temperatures in the array of sensors.
Overall the desired absolute selectivity and accuracy required in the detection of target gases is not achievable with a single approach using the known oxide film gas sensors. For a set of target combustible gases in common ambient background interference, there is a need to use means other than the film composition and temperature to minimize the effect of the interference gases on the combustible gas response of the sensor. Further there is need to address the fact of reaction rate differences between certain target gases such as methane and carbon monoxide and some common "interference" gases, such as ethanol with a tin oxide based film. Overall there is a need to provide a distinct sensor response to different target gases and to minimize the effect on the film resistance response to interference gases.
It is also known, as disclosed in U.S. Pat. Nos. 4,457,161 and 4,481,499, that gas sensors formed from different kinds of metal oxide materials have different gas selectivities to specific gas components of a mixed gas. Therefore there is need to provide a film composition that exhibits favorable catalytic activity with respect to solvents so that the effects of interference from gases other than the target gases are minimized while the sensitivity to target gases is improved. Accordingly an array of thick films that produces the preferred carbon monoxide and methane signals will enhance the accuracy in the calculated concentrations of these target gases.